Patent GB-674203-A (GB)
Title: Process and apparatus for the production of porous foils from viscose
IPC: C08L1/24
Filed: 1950-02-06
Abstract: <PICT:0674203/IV (a)/1> In the manufacture of porous foils, a viscose, which has been prepared by the xanthation of an unaged or only slightly aged alkali cellulose with 45-60 per cent of carbon disulphide calculated on the weight of cellulose present in the alkali-cellulose and has undergone no ageing or ageing not exceeding a Hottenroth (ammonium chloride) number of 20, and has a relatively high cellulose content, is mixed to form a pasty mass with at least 40 per cent, calculated on the weight of cellulose in the viscose, of a substance in the form of loose staple fibres, which functions as a reinforcing medium, and with 8-30 times the weight of the combined cellulose in the viscose of a granular inorganic or organic pore-forming compound, the grains of which all pass a sieve with circular apertures of 0.5 mm. diameter, do not cause premature coagulation of the viscose and can be dissolved out of the foil, after the latter has been formed, so as to leave pores therein, the mixture is distributed between two plates, bands or like members, the surfaces of which in contact with the mixture are smooth and may also be patterned to give an imprint free from sharp edges, pressed to the desired thickness and heated to decompose the cellulose xanthate contained therein whilst still in contact with at least one surface, whereafter the foils so formed are washed with a solvent for the pore-forming compound and dried.  Dyestuffs or pigments may first be added to the viscose, e.g. in a proportion of 0.5-2 per cent of the weight of combined cellulose in the viscose.  In order to make the product flexible, a natural resin or a synthetic resin such as polystyrene may finally be added to the paste in solution or emulsion form in proportions which do not exceed those of the cellulose component of the viscose.  In an example an alkali cellulose which has been only slightly aged is xanthated with 45 per cent carbon bisulphide calculated on cellulose, and dissolved in dilute aqueous caustic soda solution to form a viscose containing 8.0 per cent of cellulose and 5 per cent of caustic soda.  7 Kgms. of this viscose, after the addition of 3 grams of Indanthrene Yellow, are mixed in a stuff grinder with 6 Kgm. of Glauber's salt, the granules of which pass a sieve with circular openings of 0.5 mm. diameter, the Glauber's salt being sifted through a shaker sieve into the viscose during the kneading.  260 grams of cotton of a staple length of about 30 mm. are uniformly distributed in the resulting paste.  Finally 25 per cent of polystyrene, calculated on the cellulose in the viscose, is added.  After intimately mixing for an hour the paste produced is placed in several portions on a heatable press plate.  The counter plate is then pressed firmly on to the lower plate in a press until the spacing is 1.5 mm.  The plates are allowed to remain firmly united and are then heated to approximately 90 DEG  C.  After two minutes the plates are separated and the foil removed and steeped in 5 per cent sulphuric acid, rinsed with water and dried. In a similar example hemp fibre is used as a re-inforcing medium and phenanthrene as a pore former.  After removal from the press the foil is treated first with water to remove decomposition products of the viscose and then with acetone to dissolve out the phenanthrene pore-former. Thereafter the foil is treated with hot water.  In an illustrated example of a continuous process 7 Kgms. of a viscose prepared as above are mixed for half-an-hour with 10 grams of ochre, 7 Kgms. of sodium acetate granules, 315 grams of finely teased jute and 6 Kgms. of a fine Glauber's salt as in example 1.  The resulting paste is filled into a screw press and extruded in the form of a band 29 (Figs. 1 and 2) through a tube 30 ending in a slot-shaped extrusion orifice approximately 15 mm. by 60 mm. on to an endless moving metal band 1 running around guide rollers 31.  A second endless band 2 of rubber, running around guide rollers 32, covers the paste band 29. Both moving bands 1 and 2, which are of equal width and are positively driven by means not shown at a speed of 1 cm. per sec., now travel over a table 3, where idler pressure rolls 4-7, acting on the upper band 2, press out the band 29 of paste into a foil. Alternate rolls 4 and 5, 6 and 7, sweep outwards from the middle of the band 2, one to each edge, while they press on the band and then return out of contact with the band to the middle again.  The rolls move outwards at an angle to the direction of travel of the band 2 with a component of velocity equal to and in the same direction as that of the band 2, so as to avoid any tendency for the paste to be dammed up at the front faces of the rolls.  An idler pressure roll 8, revolving about an axis at right angles to the run of the bands 1, 2, then presses the paste out further.  Lateral expansion of the foil is limited by a pair of guide strips 9 arranged behind the pressure roll 8 and mounted on the table 3 in such a position as to extend inwards over the margins of the bands 1 and 2 and to dam back the paste and cause the reinforcing fibres to be oriented parallel with the margin of the foil.  The following idler rolls 10 and 11, which are parallel with the roll 8, press out the foil uniformly to a thickness of 1.5 mm.  The foil and the bands 1 and 2 then enter the decomposition chamber 12 heated to 80 DEG  C.  On leaving this chamber the bands 1 and 2 part from the foil and the latter passes over a guide roll 13 into a vat of dilute sulphuric acid 14.  The foil passes through this vat and is squeezed by a number of pairs of squeeze rolls such as rolls 15 and 16.  The foil then passes through a pair of squeeze rolls 17, 18 into a water vat 19, in which it passes through a number of pairs of squeeze rolls such as 20 and 21.  The foil passes between a pair of rolls 22 and 23 into a vat 24 containing a softening bath, e.g. a 30 per cent aqueous solution of glycerin.  In the vat 24 the foil runs between squeeze rolls 25 and 26 and passes by way of final rolls 27 and 28 into a drying chamber, and is subsequently reeled.